In network communication, a comparatively small-sized network sometimes makes a connection to a large-sized network dynamically using an apparatus for connecting networks such as a gateway so as to send/receive data to/from the large-sized network.
For example, the dynamic connection to a network corresponds to a case of making a connection to a network provided by an ISP (Internet Service Provider) through a dial-up router from a home-LAN (Local Area Network). The user has an access to an ISP from a PC (Personal Computer) configuring a home-LAN through dial-up accessing. Then, the ISP allocates an IP (Internet Protocol) address to the PC. Thus, the home-LAN and a network provided by the ISP is connected through a dial-up router working as a gateway. The IP address allocated to the PC by the ISP is changed every time dial-up accessing is made.
In network communication, IP addresses of respective hosts existing in a network, which specify communication partners, have to be different from each other, and global IP addresses being unique IP addresses are given to respective hosts on a network under management of the NIC (Network Information Center) and JPNIC (JP-Network Information Center).
A closed network such as a home-LAN or a firm-LAN is not subject to the rule, and independent IP addresses can be set up so long as overlapped addresses are not used in the closed network. In such a LAN, private IP addresses, which are not permitted on the Internet, can be used. In case a private IP address is sent onto the Internet, its packet is deleted since there is made a stipulation that such a private IP address must not be routed.
In a firm, as described above, a LAN may be configured using private IP addresses, and, for example, a main office and its branch offices may independently configure a LAN respectively.
When performing network communication by connecting LANs that are independently configured using private IP addresses, there occurs no problem in case respective used address spaces are different from each other. On the other hand, in case respective used address spaces are overlapped, hosts of the same address undesirably exist in respective networks, which disallows communication through a gateway.
In case there exist hosts of the same address, a private IP address of a host belonging to one of networks has to be set up manually, which requires troublesome work.
It is anticipated that private IP addresses will further be used by an ISP (Internet Service Provider) so as to avoid risks due to full-time connection, or owing to depletion of finite IP addresses. Thus, as described above, when making a connection between different networks, there often occur address overlaps, which undesirably leads to delayed communication between networks.